Hi, I hear that light always seems to travel at c, but I've failed to completely understand what that means, could anyone help out with that? c is measured in m/s, but both seconds and meters seem to be able to change depending on the observer due to time dilation and length contraction. When talking about m/s are we talking about earth seconds, or the observers seconds? And are we talking about earth meters, observers meters, universal meters or light meters? Is the constant speed only about how quickly the observer sees light arrive or pass by, or is it also about how an observer would experience light travelling along the entire path? If a observer travelling at 99.9999% the speed of light along the same path would measure the light arriving, which variables would be the same as compared to an observer which is static? Which would be different if any? I thought that this experiment could help me make more sense of it all: Place two lightsources right next to each other, both at a distance of 299792458 meters away from the points where some equipment will be. Both the lightsources and the points are the same distance apart of each other. All clocks mentioned were synchronized to a time at the start, and have been moved in such a way that no dillations have occurred that would cause them to lose their sync. Both lightsources fire off a pulse towards their respective points, both in the same direction, which is done at T0. Put a static observer at one of the points, this static observer receives the light signal and notes the time T1. Take a second observer moving at half the speed of light, starting at 149896229 meters away from the same point from the same direction as the light came from, which is also being released at t0. When the light hits this observer, it notes T2 on its own clock, but also sends a quick light pulse to a stationary clock right next to it (assume distance = 0, or very close to it) , which notes T3 as soon as this lightpulse hits. What can now be said about any of these times? Is T1 - T0 a second? Is T1 the same as T2? Is T1 the same as T3? Would both of the observers see the light arriving at 299792458 m/s? Are there any other interesting things that could be said about the results, or any more interesting results that could be found if the experiment was tweaked somehow? Thank you for your time.